Addressing a party gathering in the central town of Kandy, United National Party (UNP) leader Ranil Wickremesinghe called for military discipline among the party cadres in order to fight Rajapaksa.
The next parliamentary election is not due until 2016 but Wickremsinghe predicts the government would not last its full term and there will be elections by 2014.
"What we are seeing today is a dictatorship. We have to strengthen the party grass roots machinery to capture power," Wickremesinghe said.
The UNP has spent all but two years in government since 1994. The party's continued election losses are being blamed on his leadership. The UNP has suffered constant defections of its members to the government since 1994.
The opposition remains divided and fragmented as Rajapaksa continued to enjoy popularity among the island's Sinhala majority community as a result of his military action which defeated the LTTE campaign for a separate Tamil state.